Turbines generally comprise a centripetal turbine wheel mounted in a turbine housing, an annular turbine inlet passageway extending radially inward towards the turbine wheel periphery, a turbine volute arranged around the radially outer end of the turbine inlet passageway, and a turbine outlet passageway extending axially from the turbine housing. The passageways and housing communicate such that pressurized gas, such as exhaust gas from an internal combustion engine, is admitted to the inlet volute and flows through the annular turbine inlet passageway to the turbine outlet passageway via the turbine chamber, thereby driving the turbine wheel. The turbine wheel drives a shaft which in turn drives a centrifugal compressor.
In a variable geometry turbine, one wall of the annular turbine inlet passageway may be provided by a moveable wall member the position of which is adjustable, relative to a facing wall of the turbine inlet passageway, to control the width of the inlet passageway and therefore its cross-sectional flow area. A variety of actuation mechanisms, including hydraulic, mechanical and pneumatic, have been proposed to control the position of the moveable wall member.
For example, U.S. Pat. No. 5,044,880 describes a variable geometry turbine in which the moveable wall member is annular and formed from a sheet material, and is mounted within an annular cavity in the turbine housing. The wall member has a tubular portion extending from the radially outermost periphery of the wall member and away from the facing wall of the housing. A gas tight seal is disposed between the tubular portion and the turbine housing to prevent gas leaking from the inlet passageway to the space behind the annular wall member. Such leakage would impair the efficiency of the turbine.
In a moving sidewall type variable geometry turbine, when the volume of exhaust gas being delivered to the turbine is relatively low, the velocity of the gas reaching the turbine wheel is maintained at a level which ensures efficient turbine operation by narrowing the annular turbine inlet passageway. Vanes extending across the inlet passageway direct the gas flow into the wheel in an efficient manner. As the passageway is narrowed, for any given volume of gas flow, the velocity of the gas passing through the turbine inlet passageway increases. As a result, the radial pressure gradient across the face of the moveable wall member, from its upstream periphery to its downstream periphery, increases as the passageway is narrowed. Assuming that the radially outer edge of the moveable wall member is sealed to prevent gas leaking behind the wall member, the pressure behind the wall member will be lower than the pressure on the radially outer portions of the face of the moveable wall member. This pressure difference results in high loads on the wall member urging it toward a more open position. For example, in a turbine for a ten liter internal combustion engine, a load of 80 kg (176 lb) can be applied to the moveable wall member when the passageway width is a minimum. When braking, the pressure difference can be as high as 5 bar, resulting in a level as high as 200 kg (440 lb). These loads must be borne by the actuation system.
In addition to the disadvantageous effect on the control of the position of the moveable wall member, the high loads produced on the face of the wall member can increase wear and increase the complexity and cost of the overall variable geometry turbine which must be constructed to handle the high loads produced.
It is an object of the present invention to provide a variable geometry turbine which obviates or mitigates the above disadvantages.